1. Field of the Invention
The present invention relates to a pulse width modulation (PWM) driving IC and a PWM output signal generating method, and more particularly, to a PWM driving IC and the PWM output signal generating method with less input pins to achieve both a direct PWM control mode and an indirect PWM control mode.
2. Description of the Prior Art
As the development of computer technology in recent years, the processing pulses of a central processing unit (CPU) increase rapidly, the heat generated from the CPU becomes more and more. Therefore, the need for heat-dissipation becomes more important as well. The main method for heat-dissipation among many those used in nowadays is still heat-dissipating fans. There are voltage control and pulse width modulation (PWM) control methods for heat-dissipating fans used in CPUs, wherein the control modes of the PWM control can be divided into a direct PWM control mode and an indirect PWM control mode.
In detail, please refer to FIG. 1A, which is a schematic diagram of a conventional PWM driving IC 10 having a direct PWM control mode. As shown in FIG. 1A, the PWM driving IC 10 comprises a PWM pin 102, for receiving a PWM signal PWMS such that the PWM driving IC 10 can directly output the PWM signal PWMS as a PWM output signal PWMout to control output and drive a fan.
For example, please refer to FIG. 1B, which is a schematic diagram of a driving circuit 104 of the PWM driving IC 10 shown in FIG. 1A. As shown in FIG. 1B, the PWM driving IC 10 can control on/off of an upper gate switch 106 and a lower gate switch 108 of the driving circuit 104 via the PWM output signal PWMout, to change a driving current IL of the fan, so as to change the rotational speed of the fan. In such a condition, please refer to FIG. 1C, which is a schematic diagram of the PWM signal PWMS and the PWM output signal PWMout of the PWM driving IC 10 shown in FIG. 1A. As shown in FIG. 1C, since the PWM output signal PWMout equals to the PWM signal PWMS, a duty of the PWM output signal PWMout and the corresponding rotational speed of the fan can be controlled via adjusting the duty of the inputted PWM signal PWMS.
However, since a rotational speed generated from the direct PWM control mode is proportional to the duty of the PWM signal PWMS, if there is a need for special applications, e.g. the PWM output signal with minimum or maximum operating duty, such as the operating duty of 20% or 80%, it needs to utilize the indirect PWM signal control mode.
Please refer to FIG. 2A, which is a schematic diagram of a conventional PWM driving IC 20 with an indirect PWM control mode. As shown in FIG. 2A, the PWM driving IC 20 comprises a set pin 202, an oscillating pin 204 and a comparator 206. The set pin 202 and the oscillating pin 204 receives a set signal SET and a triangle wave OSC, respectively. The comparator 206 compares the set signal SET with the triangle wave OSC to generate a PWM output signal PWMout′, wherein the set signal SET relates to the duty of the inputted PWM signal PWMS. In such a condition, the PWM driving IC 20 can control output to drive the fan via the PWM output signal PWMout′ according to the PWM signal PWMS indirectly, wherein the driving method is similar to that shown in FIG. 1B.
For example, by switching on/off of a bipolar junction transistor (BJT) Q1 via the input PWM signal PWMS plus, the DC set signal SET inversely proportional to the duty of the inputted PWM signal PWMS can be generated by a voltage circuit 208 and a filter circuit 210, i.e. during on-time of the PWM signal PWMS, the BJT Q1 is turned on and the resistances R2 and R3 are connected in parallel, so a voltage cross the resistance R3 is less.
In such a condition, please refer to FIG. 2B, which is a schematic diagram of the set signal SET, the triangle wave OSC and the PWM output signal PWMout′ shown in FIG. 2A. As shown in FIG. 2B, the comparator 206 can compare the DC set signal SET with the triangle wave OSC generated from an oscillator 212 to generate the PWM output signal PWMout′. As a result, the relation between the set signal SET and the duty of the inputted PWM signal PWMS is decided via adjusting the resistances R1-R3 of a voltage divider circuit 208, so as to decide the minimum or maximum operating duty of the PWM output signal PWMout′ and the corresponding minimum or maximum rotational speed.
For example, when the duty of the PWM signal PWMS is 0%, the voltage of the set signal SET equals to a voltage cross the resistances R2 and R3. At this moment, if resistances of the resistances R2 and R3 are greater, the maximum of the set signal SET is greater, such that the minimum operating duty and the corresponding minimum rotational speed of the PWM output signal PWMout′ are less, so as to adjust the minimum operating duty of the PWM output signal PWMout′ and the corresponding minimum rotational speed. Therefore, the maximum operating duty of the PWM output signal PWMout′ and the corresponding maximum rotational speed can be adjusted by the same token.
Noticeably, in order to make both the duty and a frequency of the PWM output signal PWMout′ adjustable, the circuit for generating the set signal SET and a capacitor cascaded to the oscillator 212 shown in FIG. 2A are both outside the PWM driving IC 20, so the PWM driving IC 20 need to include the set pin 202 and the oscillating pin 204.
On the other hand, please refer to FIG. 3, which is a schematic diagram of a conventional PWM driving IC 30 with both a direct PWM control mode and an indirect PWM control mode. As shown in FIG. 3, if the prior art tends to have both the direct PWM control mode and the indirect PWM control mode, the PWM driving IC 30 must comprise 3 input pins 302-306 for receiving the PWM signal PWMS, the set signal SET and the triangle wave OSC to perform the operations of the PWM driving IC 10 and 20, respectively.
However, due to limited pins of a general PWM driving IC, if the conventional PWM driving IC realizes both the direct PWM control mode and the indirect PWM control mode with 3 pins, the functions of the PWM driving IC are limited. Therefore, there is a need to improve the prior art.